Hypertension is generally defined as the elevation of the systolic and/or diastolic arterial blood pressure above a nominal value of 140/90 mm Hg. Diseases associated with hypertension include arteriosclerosis, hypertensive renal failure, stroke, congestive heart failure and myocardial infarction. Although numerous methods of treatment have been found to be effective in the reduction of arterial blood pressure, the etiology of essential hypertension remains essentially unknown. A genetic predisposition to hypertension is generally accepted, but the number of different drugs which have been found effective in the treatment of hypertension, and the fact that these drugs seem to operate by eliciting different pharmacological responses, suggests that there may be different primary causes for essential hypertension.
A number of studies have suggested that one or more circulating factors may play a role in the genesis or the maintenance of hypertension [See: Wright et al., A Hypertensive Substance Found in the Blood of Spontaneously Hypertensive Rats; Life Sci. 1984; 34:1521-1528; Dahl et al., Humoral Transmission of Hypertension: Evidence from Parabiosis; Circ. Res. 1969; 24/25 (Suppl. I):21-23; Greenberg et al., Evidence for Circulating Factors as a Cause of Venous Hypertrophy in Spontaneously Hypertensive Rats; Am. J. Physiol. 1981; 241:H421-H430; Tobian et al., A Circulating Humoral Pressor Agent in Dahl S Rats with Salt Hypertension; Clin. Sci. 1979; 57:345s-347s; Zidek et al., Humoral Factors in the Pathogenesis of Primary Hypertension; Klin. Wochenschr. 1985; 63 (Suppl. II) D:94-96; Hirata et al., Hypertension Producing Factor in the Serum of Hypertensive Dahl Salt-Sensitive Rats; Hypertension 1984; 6:709-716]. For example, in parabiosis and cross-circulation experiments, an increase in blood pressure could be induced in normotensive animals by exposure to blood from hypertensive animals. The subcutaneous injection of erythrocyte-associated factor obtained from spontaneously hypersensitive rats (SHR) has been shown to induce hypertension in normotensive Wistar-Kyoto (WKY) rats and an increase in blood pressure can be induced in normotensive, salt sensitive Dahl rats by injection of serum from hypertensive, salt-sensitive Dahl rats.
There have also been reports of circulating factors in both hypertensive rats and hypertensive humans which increase intracellular calcium [See: Banos et al., Two Factors Associated with Increased Uptake of Calcium in Platelets from Essential Hypertensive Patients; Clin. Exp. Hypertens. 1987; 9:1515-1530; Zidek et al., Effect of Plasma from Hypertensive Subjects on Ca Transport in Permeabilized Human Neutrophils; Clin. Sci. 1988; 74:53-56; Linder et al., Effects of a Circulating Factor in Patients with Essential Hypertension on Intracellular Free Calcium in Normal Platelets; N. Enc. J. Med. 1987; 316:509-513; Bruschi et al., Cytoplasmic Free Ca is Increased in the Platelets of Spontaneously Hypertensive Rats and Essential Hypertensive Patients; Clin. Sci. 1985; 68:179-184; Wright et al., Stimulation of Aortic Tissue Calcium Uptake by an Extract of Spontaneously Hypertensive Rat Erythrocytes Possessing Hypertensive Properties; Can. J. Physiol. Pharmacol. 1986; 64:1515-1520]. Since vascular tone is influenced by the level of intracellular calcium, it would seem reasonable to assume--although it has not yet been experimentally shown--that factors which increase blood pressure and factors which increase intracellular calcium may be related. There has been accumulating evidence suggesting the involvement of calcium regulating hormones in some forms of hypertension [See: L. M. Resnick, Am. J. Med. 82 (Suppl. 1B), 16 (1987)]. Parathyroid hormone (PTH) is a calcium regulating hormone. Thirty percent or more of essential hypertensive patients fall into a subgroup characterized by increased levels of immunoreactive parathyroid hormone (ir-PTH). [See: Laragh et al., Kidney Int. 34, (Suppl. 35), S162 (1988)]. An increase in PTH levels has been reported in SHR rats [See: McCarron et al., Hypertension 3 (Suppl. 1), I162 (1981)] and it has been observed that hyperparathyroid patients often exhibit hypertension, the severity of which can, in most cases, be reduced by parathyroidectomy [See: Hellstrom et al., Brit. J. Urol. 30, 13 (1958)]. Similar results from parathyroidectomy have also been reported in SHR rats. [See: Schleiffer et al., Jap. Circ. J. 45, 1272 (1981)]. Various investigators have suggested that PTH contributes to the development of essential hypertension, although exogenous administration of PTH causes a reduction in blood pressure in mammals and other vertebrates [See: Pang et al., Gen. Comp. Endocrinol. 41, 135 (1980)]. This vasodilating action of PTH also has been related to a specific region of the molecule separate from the region mediating hypercalcemic effects [See: Pang et al., Endocrinology, 112, 284 (1983)]. pTH has also been shown to inhibit calcium entry into vascular smooth muscle [See: Pang et al., Lifr Sci., 42, 1395 (1988)] through L-type calcium channels [Wang et al., submitted for publication]. This paradox is further heightened by the fact that hypertensive patients with increased PTH levels exhibit decreased serum ionized calcium levels [See: Resnick et al., New Encl. J. Med., 309, 888 (1983); Hvarfner et al., Acta Med. Scand., 219, 461 (1986)]. It would be expected that the serum ionized calcium levels would be elevated if PTH were primarily elevated.
The involvement of the parathyroid gland in essential hypertension has been apparent but existing literature on the action of PTH on the vasculature is not consistent with a causative role for PTH in essential hypertension. At the time that this invention was made, PTH was the only active hormone reported to be produced by the parathyroid gland.
Calcium channel blockers were identified as a method for the control of hypertension, as reported by Fleckenstein et al., Z. Kreislaufforsch, 56, 716 (1967), and are routinely used in the control of hypertension. Three calcium channel blockers are currently of clinical significance in the United States, verapamil, nifedipine and diltiazem. All three achieve their anti-hypertensive effect by inhibiting the entry of calcium ions into vascular smooth muscle. The ultimate effect is vasodilation calcium channel blockers, by limiting the uptake of calcium-in vascular smooth muscle, are beneficial, but have been found to stimulate some endocrine systems, such as the RAS system. [Kotchen et al., Am. J. Cardiol., 62 41G (1988); Matsumara et al., J. Pharmacol. Exp. Ther., 241, 1000 (1978); Resnick et al., Fed. Proc., 45, 2739 (1986)]. Utilization of calcium -channel blockers may be limited by excessive vasodilation, negative inotropy, excessive depression of the sinus nodal rate, atrial-ventricular nodal conduction disturbances and interference with non-vascular smooth muscle contraction. A combination therapy which minimizes the amount of calcium channel blocker required to achieve the desired anti-hypertensive effect is desirable.